In order to stabilize and steer his aircraft, a helicopter pilot manually activates piloting means (cyclic stick, collective pitch and pedals) in order to act on the piloting axes of the helicopter (main rotor or tail rotor). A lateral or longitudinal movement of the cyclic stick makes it possible to act respectively on the lateral or longitudinal axes of the helicopter by modifying the incidence of the main rotor blades. The collective pitch makes it possible to adapt the engine power to the flight conditions by modifying the incidence of the main rotor blades and makes it possible to modify the ascent speed. The pedals make it possible to orient the nose of the helicopter by modifying the incidence of the tail rotor blades. The movement of the piloting means is transmitted to the piloting axes by means of mechanical transmission systems which are constituted by various mechanical relays. The assembly constituted by a piloting means and an associated transmission system constitutes a flight control linked with the piloting axis in question. The end of the flight control in contact with the piloting means is called the output end of the flight control.
Helicopters are often equipped with automatic piloting equipment which acts on the flight controls, under the authority of the pilot, in order to carry out two main tasks: a first task of assisting the pilot and a second task of automatic piloting.
When it is assisting the pilot in the manual control of his helicopter, the automatic piloting equipment makes it possible, on the one hand, to dampen the movements of the machine in order to facilitate its control by the pilot and, on the other hand, to maintain the current flight configuration (lateral and longitudinal attitudes and heading) thus making it possible for the pilot to release the piloting means momentarily without going into a flight configuration which would be dangerous.
When it is in automatic piloting mode, the automatic piloting equipment makes it possible to slave one or more flight parameters (lateral speed, heading, navigation, etc) to one or more instruction values chosen previously by the pilot.
In order to act on a flight control, the automatic piloting equipment uses actuators. Among these actuators are distinguished actuators called “trim actuators” and actuators called “series actuators”. The “trim actuators” are actuators, with or without return force, which have a large travel covering all of the positions of the flight controls but whose response time is relatively slow. They are generally of the rotary type and are placed in parallel with the flight controls.
“Series actuators” are mechanical actuators which are placed in series with the flight controls, they comprise a body and an output shaft, they are generally of the endless screw/nut type and have reduced authority and a short response time. They convert an electrical command into a movement of translation of their output shaft with respect to their body. “Series actuators” are called “mechanically irreversible”. That is to say they deform solely when an electrical command is applied to them. In particular, when the automatic piloting equipment is not operating, the “series actuators” are without effect on the control of the control of the helicopter. A neutral position of a series actuator corresponds to the position where the free end of its output shaft is at mid-travel.
For safety reasons, the series actuators are duplicated in the flight controls in order to provide redundancy and the same thing applies to the calculating chains of the trim actuators.
The low authority of a series actuator necessitates a servo-control of the movement of the trim actuator so that the series actuator is at all times close to its neutral position. A control device defines the command to transmit to the trim actuator. In general this command is electrical and comprises information on the direction of the rotational movement and on its desired speed. A control device comprises two calculation chains, each fed with data on the position of the output shaft of one of the series actuators and a circuit for mixing instructions which generates a command from the instructions coming from the different calculation chains. In the prior art, a first chain called “supervision” delivers only a direction of movement instruction, a second chain called “control” provides both a direction of movement instruction and a speed instruction. In order to generate a primary movement command, the circuit for mixing instructions controls only the direction of movement: it prohibits a movement of the trim actuator in a direction which is not in accordance with the instruction from the supervision chain. Such a control device architecture is sufficient for the piloting of aircraft that are not very reactive.
When a sudden failure of the “control” calculation chain occurs, during a transient period starting from the instant when the failure is initiated up to the instant when the “supervision” chain modifies the direction of movement of the actuator instruction, an incorrect command arrives at the actuator and acts upon its movement. Certain aircraft models, and this is the case of certain helicopters, are very reactive and even relatively unstable, and a failure of this type on such an aircraft, particularly if the incorrect command is of high amplitude, for example a roll command at a speed of 30 degrees per second, can prove to be dramatic to the pilot of the aircraft, since the pilot's reaction time is too slow to allow him to compensate, by himself, for the effects of the incorrect command during this transient period.